Seismic data acquisition surveys include both land and seabed surveys that utilize seismic receivers arranged in a pattern or grid on either the land or seabed. Seismic sources or seismic shots are created by towing or driving one or more seismic signal generators such as a seismic gun along tow lines or paths, e.g., shot lines. The seismic signal generators are then actuated at multiple locations along the tow lines or paths and the resulting seismic signals are recorded at the seismic receivers on the cables or nodes. The recorded seismic signals are then processed to yield a seismic image of the subsurface below the seismic data acquisition grid and to generate data such as the elastic properties of the subsurface.
A major challenge in seismic characterization of the subsurface is establishing a quantitative link between variations in pore geometry within the subsurface and changes in the elastic properties of the subsurface especially in carbonate rocks which tend to have complex pore geometry. Inclusion based theories are commonly used to address the issues resulting from variations in pore geometry. The inclusion based theories model wave velocity and attenuation based on scattering theory, approximating the rock as an elastic block of mineral perturbed by holes (porosity). In general, the volume fraction of the constituents and the physical and geometrical properties of the constituents alone and relative to each other are utilized in determining a solution.
An important aspect of the geometrical properties of the constituents relates to pore structure. Pore structure is typically used to include the effects of changes in the pore shape through a parameter referred to as the “pore aspect ratio”. This parameter assumes pores as an ideal ellipsoid having a short axis and a long axis and is defined by dividing the length of the short axis by the length of the long axis. A common practice is to approximate the inclusion shape with a single spheroid with one optimized pore aspect ratio. Pore aspect ratio is often used to characterize the pore-space geometry in rocks, and its application ranges from hydrocarbon reservoir characterization to environmental issues. Inclusion-based rock physics theories, which are normally used to include pore structure into elastic properties, show strong dependence on the choice of pore aspect ratios. Therefore, even small changes in the aspect ratio result in different elastic properties using inclusion models.
The assumption of an ideal ellipsoid for pore shape is not a valid assumption when pore geometry deviates from a simple shape. The result is an increase in modeling error with increasing complex pore geometries regardless of the accuracy on other inputs into the inclusion model. This type of complex pore geometry is more common in sediments with secondary porosity like carbonates where diagenesis forces pores to evolve as arbitrary shapes which may not be fully approximated by an ellipsoid.